Ultraviolet light has a degradative effect on olefin polymers, the severity of which is dependent on the particular polymer and the geographical location of exposure. The degradation may take the form of discoloration, loss of tensile and impact strength, distortion of initial flexibility, dimensional change, surface craze, cracking, powdering or increased electrical conductivity. All of these effects may result from the breaking of carbon-to-carbon bonds in the polymer chain followed by immediate oxidation of the chain fragments.
It is well known that the addition of certain materials to an olefin polymer will impart a degree of stabilization to that polymer with respect to its resistance to the destructive forces of ultraviolet radiation. These materials, in one instance, function as preferential acceptors of incident ultraviolet radiation because they have a much higher affinity for such radiation than does the olefin polymer. It appears that they absorb harmful radiation and dissipate it as harmless energy. They thus form a protective shield for the polymer in which they are present.
Although there are many types of ultraviolet stabilizers, an important class includes the 2-hydroxyphenylbenzotriazoles. They offer strong intensity and broad ultraviolet stabilization with a fairly sharp wavelength limit close to the visible region. They absorb light at 280-390 mm. which is the spectral region most harmful to polymer claims.
Olefin polymers which are stabilized by an ultraviolet stabilizer frequently contain also an oxidation inhibitor. This is especially so where it is important to achieve light stable polymer compositions. A wide variety of antioxidants are available.
U.S. Pat. No. 4,206,111 (Valdiserri et al.) shows the combination of certain 2-hydroxyphenylbenzotriazoles, dialkylpentaerythritol diphosphites and phenolic ester oxidation inhibitors. The combination is an effective ultraviolet stabilizer in olefin polymers, being effective to extend the useful life of polypropylene, for example.
U.K. Pat. No. 1,526,603 shows a process for preparing bis-(dialkylphenyl)pentaerythritol diphosphates; also shown is the use of such disphosphites in combination with a phenolic inhibitor to impart an enhanced thermal stability to polypropylene.